1. Field of the Invention
The present invention relates to combustibles detectors generally and particularly to differential thermocouple devices modified to detect combustible gases.
2. Description of the Prior Art
Combustible gases detectors are known which utilize electrically heated platinum filaments exhibiting resistance changes in response to variable concentrations of combustible gases. The resistance change of such devices is notoriously small requiring their use in a Wheatstone bridge arrangement. This arrangement increases the cost of the device. An example of such a device may be found in U.S. Pat. No. 3,092,799 issued June 4, 1963 to A. R. Baker.
Along with the required use of a Wheatstone bridge these known devices have other shortcomings. A constant current power supply or other regulated excitation source is required and the devices exhibit drift rates which are too large to permit industrially stable operation at low combustibles concentrations. Part of the drift problem comes about because the nominal resistance value of these sensors is the order of 1.00 ohm and the presence of combustibles causes typical resistance increases to 1.04 ohms. Resistance changes of this magnitude are too close in size to the kinds of changes experienced due to fluctuations in contact resistance, electromigration, grain growth, etc.
Differential thermocouples are also known for use in thermal analysis as evidenced by U.S. Pat. No. 3,491,581 issued Jan. 27, 1970 to F. E. Roberts. However the applicant is unaware of any prior art device using such differential thermocouples coated with a catalyst and a non-catalyst to provide a self-powered, inexpensive and stable combustible gases detector.